[Wannier] Boltzwann problem order of magnitude electrical conductivity

[Robert Benda]

Dear all Wannier users, 

I am quite new to Wannier90 and I am mostly performing electrical conductivity calculations using the BoltzWann module. I have followed the tutorial (example 16) on the computation of Boltzmann conductivity tensor for Silicon. 

I attached the results I obtained for a 10 fs relaxation time as ' boltz_relax_time ' Boltzwann parameter. 

The order of magnitude of the largest conductivity component is 10^5 to 10^7 S/m, if I was not mistaken in the previous calculations using Quantum Espresso (I used the default parameters provided in the example16/ repository as input filesfor SCF and NSCF calculations). 

I am very surprised, as I was expecting something of order 10^{-3} S/m, for the conductivity of Silicon, which is a semiconductor. Increasing more the relaxation time to 100 fs roughly produces a tenfold increase, accordingly to the scaling factor of the relaxation time, on the conductivity, which happens to be even larger, or order 10^8 S/m ; and farther away from the experimental conductivity. 

In all cases, as far as I know, relaxation times in most solids (metals/semiconductors) range from a few fs to about 1000 fs -- that high, at low temperature and for some low dimensional materials-- and in this range I cannot reproduce a conductivity of order 10^{-3}, about 10 orders of magnitude smaller than what I found. 

Also for a carbon nanotube (which I am interested in in the end, comparing the BoltzWann room T conductivity for defected CNTs / CNTs with adsorbed ions, to the conductivity of the perfect, pristine CNT) provided in example15, I obtain the same order of magnitude for the Boltzmann conductivity of the (5,5) metallic CNT of about 10^6 S/m.(while rather expecting from 0.5-5 * 10^{2} S/m). 

Most probably I am missing a very basis subtlety in the input parameters or the code using, that prevents me from recovering the correct orders of magnitude. That is why I kindly request your help. 

Thank you very much for your help, 
Best regards, 

Robert BENDA 
PhD student, CERMICS (ENPC) and Ecole Polytechnique (FRANCE) 


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